USING A PITOT STATIC TUBE FOR VELOCITY AND FLOW RATE MEASUREMENT 1. Overview In this article, use of a Pitot Static tube, in conjunction with a manometer will be explained. Reference will be made to the Flowinetics!" ##$ F! series manometers, as these instruments %reatl& simplif& velocit& ac'uisition. !he Pitot Static tube allows the direct measurement of d&namic pressure allowin% calculation of the %as velocit& in ducts, pipes wind tunnels etc. (. "easurement of )elocit& )elocit& * Pitot Pitot Static tube is shown in Fi%. 1.
Fig. 1 Generic Pitot Sttic t!"e con#ig!rtion. !he Pitot Static tube measures the total pressure +or impact pressure at the nose of the Pitot tube and the static pressure of the %as stream at side ports. !he difference of these pressures, i.e. the d&namic or velocit& pressure +Pd&namic varies with the s'uare of the %as velocit&. velocit&. !hus the %as velocit& ma& be expressed as-
where r is the %as densit& and $ is a correction constant dependent on the desi%n of the Pitot Static tube. O!/- !his !his e'uation is t&picall& valid for incompressible +constant densit& flow. 0i%h velocities +) will lead to increasin% errors as shown in !able (.
hen selectin% a Pitot Static tube to be used in conjunction with the F! Series +or an& manometer for that matter, it is necessar& to select a tube with a constant close to unit&, if errors in velocit& are to be avoided. If data for a particular Pitot tube is not available,
Di$tnce #ro% Pitot Sttic T!"e "$e o# ti&' or centre()ine o# *ertic) $te% to $ttic +o)e$' Di%eter ,-D Fig./ E##ect o# $ttic &re$$!re +o)e )oction #ro% Pitot Sttic T!"e $te% or ti&
the constant $ ma& be estimated. !his constant is dependent on the spacin% of the Pitot tubes2 static pressure ports +see Fi%. 1 from the base of the Pitot tube2s tip and the stem2s center line. Prandtl t&pe Pitot tubes t&picall& have constants $ close to 1. Fi%ure ( shows the effect and error of the location of the static pressure tappin%s on the static pressure error. !he lower line %ives the static pressure error associated with the distance of the static ports from the base of the tip, expressed in diameters. !he upper line presents the static pressure error due to the distance of the static ports +expressed in diameters from the stem center3line. !he use of Fi%. ( to find the constant $ for a %iven Pitot Static tube will be illustrated with an example. /xample * standard round nose Pitot Static tube has static orifices located (4 from the base of the tip and 154 from the stem2s center3line. hat is the correction constant $ From Fi%. (, the tip error is 31.67 and the stem error is 85.97. !he net error is 35.:7. !hus the indicated d&namic pressure will be too hi%h. !he correct d&namic pressure and velocit& is then-
!o simplif& determination of the constant $, !able ; ma& also be used, which shows the constant for various Pitot tube %eometric variations +for a standard r ound junction tube.
T")e 0 Pitot Sttic t!"e correction con$tnt C
!he velocit& indicated b& the F! Series manometer would then be corrected b& multiplication b& $ + for a non3unit& Pitot Static tube. Ting Me$!re%ent$ 2it+ t+e F3T Serie$ !o measure velocit& with the instrument with the %reatest accurac&, it is necessar& to measure the tar%et %ases absolute pressure and temperature as well as Relative 0umidit&, to allow the F! Series to calculate the correct %as densit&. !his is achieved b& connectin% a len%th of Silicon or !&%on< tubin% from the Pabs port to a wall static pressure tap +or avera%in% rin% at the measurement point location. *lternativel&, the Pabs port ma& be connected to the static port of a Pitot Static tube, provided $ = 1 for the tube. !emperature>R0 is measured b& partiall& insertin% the temp>R0 sensor into the duct>wind tunnel etc. "easurement starts with attachment of Silicon or !&%on< tubin% to the Pitot Static tube and the pressure transducer of choice. !he ?P8? connection barb of the transducer is connected to the !otal pressure port of the Pitot tube, and the Static pressure port of the Pitot tube is connected to the transducers ?P3? barb connection, see Fi%. 1 and the picture below. !he appropriate transducer for the expected velocit& ran%e should be used for maximum accurac&. 0owever, if in doubt as to the expected velocities, use the lar%est pressure ran%e available to avoid overloadin%. If usin% the F! (4P1*3$ Series +which accounts for compressibilit& and displa&s accurate velocities up to approximatel& (@5m>s, the ratio of the specific heats, %, must be set.
!he Pitot Static tube can then be carefull& inserted into the %as flow. It ma& be necessar& to drill holes into the ductin% for insertion. !he absolute pressure and temperature>R0 must be measured simultaneousl& with the differential pressure measured b& the Pitot Static tube for best accurac&. * ?!? tubin% barb can be used to connect the static port of the Pitot Static tube to the P3 port of the differential pressure transducer as well as the Pabs absolute pressure transducer, see the sAetch below. * Pitot Static tube with $ of approximatel& unit& should be used when this t&pe of connection is emplo&ed.
In man& applications, the ambient densit& ma& be close to the tar%et %as densit&. !his can readil& be determined usin% the F! Series b& recordin% the ambient densit& +displa&ed continuousl&, followed b& the tar%et %ases densit&. !he densit& will be calculated and autonomousl& presented b& the F! Series throu%h measurement of absolute pressure, temperature and R0. If the densit& is comparable, then simultaneous measurement of tar%et flow densit& is unnecessar&, i.e. the temp>R0 sensor can be left in its housin%.
;. Pitot Static tube duct surve&s If avera%e duct velocities, or mass or volumetric flow rates are re'uired, it is necessar& to perform a Pitot traverse of the duct. !his involves taAin% measurements at various positions across the duct. Before a traverse is conducted, it is necessar& to select a suitable location to perform the surve&. If possible, avoid traverses close to fans, dampers pipe bends, expansions etc. !r& to surve& at least 9 duct diameters downstream of the aforementioned elements and (
duct diameters upstream of these elements. !he surve& is performed with the aid of Fi%. ;. /ither the $entroids of /'ual *reas or #o%3!cheb&cheff point distribution ma& be used. * surve& proceeds as follows1.
4ecide on the number of surve& points and then marA these on the Pitot tube usin% a marAer or adjustable sprin% clips +present on some Pitot Static tubes.
(.
*t the selected surve& location, drill two perpendicular holes in the duct +for a round duct or the desired number of holes for a rectan%ular duct, ensurin% sufficient hole clearance to safel& insert the Pitot Static tube.
;.
Partiall& insert the temperature>R0 sensor in an additional hole located close to the previousl& drilled holes.
6.
$onnect Pabs to a static pressure tap>rin% close to the surve& location, or use a ?!? barb to connect to the static Pitot tube port, see sAetch above.
@.
$arefull& insert the Pitot Static tube into the duct and position at the first traverse location. /nsure that the Pitot Static tube is ali%ned with the axis of the duct usin% the ali%nment %uide on the tube as a reference.
:.
ait for the readout on the displa& to stabiliCe. If the readout continues to oscillate increase the dampin% +4*"P. If the ma%nitude of the oscillations is %reater then (@7, then another measurin% point should be considered as the results ma& not be representative.
D.
hen stabiliCed, record the desired readin%+s.
9.
"ove the Pitot Static tube to the next traversin% point and repeat @ and D until the traverse is complete.
E.
Repeat points @39 for the other traverse locations.
Once the traverse has been completed, the volumetric and mass flow rate throu%h the duct can be calculated as follows)olumetric flow rate +-
where *duct is the duct cross sectional area. n is the number of points +total number of points surve&ed. )i is the indicated velocit& at each measurement point. !hus, usin% a $entroids of /'ual *reas or #o%3!cheb&cheff point distribution allows the velocit& measurements to simpl& be summed and avera%ed. "ass flow rate + m-
where- p is the densit& of the %as in the duct. O!/- *ssumin% full& developed turbulent flow with low air swirl +rotation, i.e. after a lon% section of duct, the avera%e duct velocit& ma& be estimated usin% a sin%le Pitot readin% at the center of the duct. !he avera%e velocit& is then approximatel& 5.E of this readin% with an accurac& of G@7.
Fig. 0 Tr*er$e &oint #or rectng!)r n4 circ!)r 4!ct$. Eit+er Centroi4$ o# E5!) Are$ or Log(Tc+e"6c+e## &oint 4i$tri"!tion$ cn "e !$e4.
